1. Field of Invention
The present invention relates to a pixel structure. More particularly, the present invention relates to a pixel structure with a U-shaped storage capacitance electrode.
2. Description of Related Art
In current society, the development of multi-media technology relies much on the progress of semiconductor devices or display apparatuses. Among displays, thin film transistor liquid crystal displays (TFT-LCD) with advantages of high display quality, good space utilization rate, low power consumption, no radiation, etc. have gradually become main stream products in the market.
A common TFT-LCD is mainly formed by a TFT array substrate, a color filter substrate, and a liquid crystal layer sandwiched between the two. The TFT array substrate is formed by a plurality of pixel structures arranged in matrix. Each pixel structure is mainly formed by a TFT, a pixel electrode, and a pixel storage capacitor. The TFT includes a gate, a channel layer, a drain, and a source, and is used as a switching element of an LCD unit. When the pixel electrode is in a selected state (i.e. “ON” state), signals are written in the pixel. When the pixel electrode is in a non-selected state (i.e. “OFF” state), the pixel storage capacitor is used to keep the level to drive the liquid crystal.
FIG. 1 is a schematic top view of a conventional pixel structure. Referring to FIG. 1, the conventional pixel structure 100 mainly includes a scan line110, a data line 120, a storage capacitance electrode 130, a TFT 140, a passivation layer (not shown), and a pixel electrode 150. The scan line 110 and the data line 120 are disposed on a substrate (not shown). The H-type storage capacitance electrode 130 is disposed on the substrate. Further, the storage capacitance electrode 130 includes two branches 130a, 130b and a central portion 130c connected therebetween. The TFT 140 is disposed on the substrate and is driven by the scan line 110 and the data line 120. The passivation layer covers the scan line 110, the data line 120, the storage capacitance electrode 130, and the TFT 140. The pixel electrode 150 is electrically connected to the TFT 140 via the contact hole CH in the passivation layer.
In the pixel structure 100, the storage capacitance electrode 130 presents an H-type structure distribution, in which the central portion 130c spans across in the center of an aperture portion of the pixel structure 100, so the aperture ratio of the pixel can be reduced. Moreover, when a rubbing process is performed on an alignment film, a poor alignment may be incurred due to the existence of the central portion 130c, resulting in the problem of light leakage generated in the center of the aperture portion of the pixel, thus further leading to a low contrast of a panel.
With the storage capacitance design remains unchanged, if it is intended to increase the aperture ratio of the pixel, referring to FIG. 2, the width of the central portion 130c may be reduced, and the widths of the branches 130a, 130b may be increased, so as to maintain the same storage capacitance. FIG. 3A is a schematic view of the relative position between the storage capacitance electrode and a black matrix after the TFT array substrate with the pixel structure of FIG. 1 and the color filter substrate are assembled. FIG. 3B is a schematic view of the relative position between the storage capacitance electrode and the black matrix after the assembly of the TFT array substrate with the pixel structure of FIG. 1 and the color filter substrate shifts. In order to simplify the drawing, in FIGS. 3A and 3B, only the inner edges B1 and B2 of the black matrix are shown, and the elements on the color filter substrate are not shown. Referring to FIGS. 3A and 3B, in the design of the black matrix, in order to avoid the side light leakage, it is necessary to keep the distance between the outer edge of the storage capacitance electrode 130 and the inner edge B1 of the black matrix to be p, and to keep the distance between the inner edge of the storage capacitance electrode 130 and the inner edge B1 of the black matrix to be r. In this manner, when the assembly of the TFT array substrate and the color filter substrate shifts along X-axis, the aperture ratio of the pixel is not affected.
FIG. 4A is a schematic view of the relative relation between the storage capacitance electrode and the black matrix after the TFT array substrate with the pixel structure of FIG. 2 and the color filter substrate are assembled. FIG. 4B is a schematic view of the relative relation between the storage capacitance electrode and the black matrix after the assembly of the TFT array substrate with the pixel structure of FIG. 2 and the color filter substrate shifts. In order to simplify the drawing, in FIGS. 4A and 4B, only the inner edges B1 and B2 of the black matrix are shown, and the elements on the color filter substrate are not shown. Referring to FIGS. 4A and 4B, when the assembly of the TFT array substrate and the color filter substrate shifts along X-axis, and if the shift is larger than r, the aperture ratio of the pixel is affected, resulting in an unstable aperture ratio.
FIG. 5A is a schematic view of the relative position of the gate, the source, and the drain under the circumstance that the exposure machine does not shift during the lithography process of forming the source and the drain. FIG. 5B is a schematic view of the relative position of the gate, the source, and the drain under the circumstance that the exposure machine shifts during the lithography process of forming the source and the drain. Compared with the source S and the drain D as shown in FIG. 5A, the source S and the drain D as shown in FIG. 5B shift downwards obviously. In this manner, the overlap area between the drain and the gate increases (i.e. a black block as shown in FIG. 5B), i.e. the value of the parasitic capacitance Cgd between the gate and the drain increases, which leads to the increase of the feed-through voltage of pixel used to drive the liquid crystal molecules and negatively influences the display quality.